Optical based imaging systems are under study with a goal of providing useful images of structures deep within a body without the use of ionizing radiation. There are four basic ways researchers have been trying to image structures in densely scattering media:
One way to image in a densely scattering medium is by measuring the time-of-flight of photons that travel in the medium and detecting those photons with the shortest travel time. Photons, which experience multiple scattering well outside the beam path, have a longer time-of-flight and may therefore be rejected. This technique has been suggested by Jarry et al. in their paper “Simulation of Laser Tomoscopy in a Heterogeneous Biological Medium”, Medical & Biological Engineering & Computation, 1986, 24, 407–414. This technique has been implemented by Takiguchi et al. as described in their paper “Laser Pulse Tomography Using a Streak Camera”, Proceedings Image Detection and Quality, July 1986, and by S. Andersson-Engels et al. as described in their paper “Time-resolved Transillumination for Medical Diagnostics”, Optics Letters, Vol. 15, No. 21, November, 1990. The technique requires sophisticated pulsed lasers, with pulse times in the picosecond to femtosecond range, and a very fast detection system. With time-of-flight systems, image resolution may be improved at the expense of signal strength.
A second way to image in a densely scattering medium is by the use of coherent light illumination and optical heterodyning detection to reject scattered light. Because of the angular response of a heterodyned detector, the detector may be made sensitive only to light that exits the tissue normal to the detector axis. This technique rejects scattered light, but there is a disadvantage of very low signal strength, since the amount of coherent light in the medium falls off exponentially with the medium thickness. This technique has been demonstrated by researchers at the Thomson CGR research labs and by M. Toida et al. in the Inaba Biophoton Project, Japan. When applied to tissue imaging, the heterodyne and time-of-flight detection techniques are limited to imaging through about 2–3 cm of tissue owing to the low signal levels of the system.
A third way to image in a densely scattering medium is by the use of an optically heterodyned detector in conjunction with sound waves projected into the medium. A system of this type is described in U.S. Pat. No. 5,174,298 to Dolfi and Micheron. Dolfi and Micheron use the fact that a sound wave projected into the medium causes the scattering structures in the medium to vibrate. Light that is scattered by the medium picks up a Doppler shift equal to the medium's vibration frequency. Dolfi and Micheron detect variations in the intensity of this Doppler shift by heterodyning the Doppler modulated light passing through the medium with unmodulated light, then selecting for the Doppler shift frequencies with electronic filters.
A related method is described in U.S. Pat. No. 5,212,667 to Tomlinson, Jr. and Tiemann, wherein coherent light is projected through a scattering medium. The light emerging from the medium is a superposition of a multitude of scattered wavelets, each of which represents a specific scattering path. These wavelets are projected onto a diffuse reflecting surface (the viewing plane of a two-dimensional photodetector array) where they interfere with each other, giving rise to a speckle pattern. By introducing a focused ultrasound pulse into the medium, the positions of the scatterers are changed at a known location (probe region) in the medium, and this causes a change in the speckle pattern. By comparing speckle images before and after the scatterers are moved, the light absorption properties of the probe region may be measured even though multiple scattering interferes with direct imaging of the region.
Despite the directionality of the optical heterodyning procedures employed by Dolfi and Micheron, subsequent scattering in other portions of the scattering medium may undesirably interfere with direct imaging of the Doppler modulated light. Dolfi and Micheron describe ways to reduce this interference. These methods reduce the interference attributable to elastic scattering effects in the medium, but inelastic scattering effects in the medium may still introduce undesirable interference with direct imaging. Furthermore, since the number of photons that travel relatively straight after initial scattering is a negligible fraction of the total number of initially scattered photons, detection sensitivity tends to be poor if subsequently scattered photons remain undetected.
A fourth method to image in a densely scattering medium is by the use of a thermoacoustic response of the medium. An example of this method is described in U.S. Pat. No. 6,292,682 to Kruger. Kruger describes methods and apparatus for measuring and characterizing the localized electromagnetic wave absorption properties of biologic tissues in vivo, using incident electromagnetic waves to produce resultant acoustic waves. The electromagnetic waves are differentially absorbed as the waves pass into and through the tissue, thereby emitting acoustic waves due to the rapid local thermal expansion of the tissue. Multiple acoustic transducers may be acoustically coupled to the surface of the tissue for measuring the acoustic waves. The multiple transducer signals may be combined to produce an image of the absorptivity of the tissue, which image may be used for medical diagnostic purposes. In some embodiments, the transducers may be moved to collect data from multiple locations in order to facilitate imaging.
However, the low-resolution images produced by the Kruger method may be limited by the source pulse width, the acoustic conversion time and the receiver aperture. The method may also be prone to image artifacts due to sampling, processing and multipath interference.